Volume curves: alarm and accessibility can't be muted

Add new "non mutable" default volume curves, use them for alarm
  and accessibility.
Volume curves that start at 1 can be muted (at 0, attenuation
  is -infinity): modify alarm and a11y curves to start at 0.

Bug: 72403580
Bug: 74985293
Test: try to mute alarms through UI and fail
Change-Id: Ieed1143f392b359198acabf986caa183536761a5